Electric power steering devices for vehicles detect the steering torque and other values generated by the steering shaft from movement of the steering wheel, calculate the current reference value serving as the steering auxiliary instruction for the motor based on that detected signal, and a current feedback control circuit calculates the current control value as the difference between the current reference value and the detected motor current value, and a motor is then driven by the current control value to apply an auxiliary force to the steering wheel.
An electric power steering device of this type utilizes a motor control circuit comprised of four field effect transistors FET 1 through FET 4 connected in a bridge as shown in FIG. 7 to make up an H bridge circuit made up of two arms, a first and a second arm; a voltage V is applied across the input terminals, and a motor M is connected across the output terminals.
Among the two sets of FET pairs comprising the two mutually opposing arms in the H bridge circuit that make up the motor control circuit, the FET 1 of the first arm (or FET 2 of the second arm) is driven by a PWM signal (pulse width modulation signal) at a duty ratio D determined based on the current control value to regulate the flow of the motor current.
The rotation direction of the motor M is controlled by turning FET 3 of the second arm on, and FET 4 of the first arm off (or FET 3 of the second arm off, and FET 4 of the first arm on) based on the current control value signs.
When the FET 3 is conducting current, there is a current flow through the FET 1, the motor M, and the FET 3, and a positive current flows in the motor M. When the FET 4 of the second arm is conducting current, there is a current flow through the FET 2, the motor M, and the FET 4, and a negative current flows in the motor M. This motor control circuit cannot simultaneously drive the FET of both arms so there is little probability of an electrical short and this circuit is widely used since it is highly reliable.
FIG. 8 shows the relation between the motor current I (current actually flowing through the motor, and is different from the detected motor current value) and the duty ratio D of the PWM signal. In other words, in a state where a steering torque is generated by turning the steering wheel, the relation between the motor current I and the duty ratio D changes to that shown by the line (a) in FIG. 8. The current reference value Iref is then calculated based on the steering torque in the control circuit. The motor current control value E which is the difference between the calculated current reference value Iref and the detection value I for the motor current that was fed back, is output to the motor drive circuit so that a duty ratio D is obtained for controlling the semiconductor devices in the motor drive circuit, and no particular problems occur.
However, after turning the steering wheel, the steering wheel then returns to a straight ahead (forward) driving position (hereafter called “steering wheel return”) due to self-aligning torque. In this state, no steering torque is generated so the current reference value Iref becomes zero. However a back electromotive force is generated in the motor so that the relation between the motor current I and the duty ratio D shifts upward by an amount equivalent to the back electromotive force as shown by the line (b) in FIG. 8. This upward shift generates a discontinuous section X in the relation between the motor current I and the duty ratio D in the vicinity of the area where the value of duty ratio D is zero.
The feedback control circuit on the other hand, attempts to calculate the current control value E, however there is no duty ratio D corresponding to current reference value Iref so that an oscillating current at an amplitude nearly matching the motor current I of the discontinuous section is output as the current control value E as shown by the line (c) in FIG. 8. This type of oscillating current not only becomes a source of noise but also interferes with feedback control stability.
To resolve this problem, the inventors proposed a method of driving a motor control circuit made up of two pairs of semiconductor devices forming an H bridge circuit of two mutually opposing arms. In this bridge circuit, a first duty ratio PWM signal determined by the current control value drives the semiconductor devices of a first arm; and a second duty ratio PWM signal determined by a function of the first duty ratio, drives the semiconductor devices of a second arm, in a structure where each arm is driven separately. In this structure, there is no discontinuity in the relation between the duty ratio D and the motor current I in the vicinity of the state where the duty ratio D value is zero, even in a state where no steering torque is generated such as steering wheel return where point p is joined to point 0 in a straight line as shown in FIG. 9. Moreover, no oscillating current is output as the current control value E so that no noise is generated and stable feedback control can be attained (See Japanese Laid Open Patent Publication No. H09-39810 (1997-39810)).
In the above structure for driving the semiconductor devices of a first arm with a first duty ratio PWM signal that is determined based on a current control circuit, and driving the semiconductor devices of a second arm with a second duty ratio PWM signal defined by a function of the first duty ratio; separately driving the respective arm eliminates discontinuities in the relation between the duty ratio D and the motor current I, eliminates noise, and improves stability. However as can clearly be understood from FIG. 9, the relation between the motor current I and the duty ratio D is switched in three stages. Eliminating the chattering that accompanies this switching is difficult and problems such as control noise and vibration occur due to this chattering. This invention therefore has the object of resolving the above mentioned problems.